The present invention relates to an ophthalmic measurement apparatus, particularly to an ophthalmic measurement apparatus that irradiates the interior of an eye with a beam of substantially visible-region laser light, and uses the light scattered from the interior of the eye to perform specific measurements.
Measurement of floating cells in the anterior chamber is of critical importance when diagnosing ophthalmic inflammations, especially malfunctions of the blood-aqueous barrier and uveitis. Conventionally, a slit lamp microscope is often used for this, with grading being accomplished via the naked eye. A photographic measuring method has also been developed to provide quantitative measurements. However, no method has yet been perfected that is readily applicable to clinical examinations.
A problem with the conventional method of naked-eye measurement is the fact that as standards of judgment vary from person to person, the data thus obtained lacks reliability. One solution in recent years has been to develop ophthalmic measurement systems whereby a beam of laser light is projected into the eye and the light scattering from the eye is detected and subjected to quantitative analysis.
JP-A-64 (1989)-2623 (corresponding to U.S. Pat. No. 5,000,562) discloses an example of a method of measuring floating cells in the anterior chamber by irradiating the eye with a beam of laser light, and then measuring the light scattering from the eye. Similarly, JP-A-63 (1988) -288134 (corresponding to U.S. Pat. No. 4,991,954) discloses an example of a technique of measuring the concentration of plasma proteins (hereinafter also referred to as "flares") in the anterior chamber.
However, in the anterior chamber of a normal eye there are so few floating cells that, depending on the degree of inflammation, it may only be possible to count them using a slit lamp microscope. Therefore in cases in which the volume of the measurement space is small and there are few cells, sometimes there will be cells in the measurement space and sometimes there will be no cells.
This means that even though there may be cells in the overall anterior chamber, a cell count of zero could be obtained if there happens to be no cells in the measurement space. When this happens, the discrepancy between such a cell count and what has been seen by a visual observation has caused some examiners to distrust the system apparatus. It is preferable, therefore, to increase the size of the measurement space, which is defined by the scanning width of the laser beam used to scan the eye.
On the other hand, the presence of large numbers of cells signifies a higher degree of inflammation, in which case there will also be a higher concentration of flares. Light scattered by these flares increases the intensity of the scattered light impinging on the photosensor component of the ophthalmic measurement apparatus. Moreover, enlarging the measurement space will cause a further increase in this received light intensity level.
The flare components form a background noise component, and in received light the effect of a high level of such flare components is to bury cell signal components. The result is a loss of reliability with respect to measured values caused by an inability to differentiate between cell signal components and noise components.